Silica surfaces play an important role in the purification and analysis of chemical and biochemical analytes. In chromatographic applications, silica matrices (e.g., comprising beads or gels) have been used for decades to separate organic compounds based on differences in binding affinities under selected solvent conditions. In recent years, applications of silica matrices have been expanded to include separating non-traditional materials, such as nucleic acids, for example.
Silica gels and beads have also been used as solid-phase supports for attaching, covalently or by adsorption, coating materials that impart unique and highly advantageous separation properties. For example, a vast number of derivatized silica gel materials have been developed for analytical and preparative standard and high-pressure liquid chromatography (HPLC) to provide high-resolution separations.
Silica surfaces have also been used in the form of glass plates, tubes, and channels, to define passageways in which sample materials migrate during chromatographic or electrophoretic separations. In many of these applications, including uses in chromatography, slab gel electrophoresis, and capillary electrophoresis, it is often desired that the silica surface be inert towards the analytes of interest so as not to interfere with the separation process. For example, glass plates and columns have been treated with blocking agents, such as dichlorodimethylsilane and other silylating agents, to block surface silanol groups which would otherwise adsorb analytes or interfere with the separation medium.
In electrophoretic techiques carried out in silica-lined channels, particularly with narrow channels, the physical condition of the silica surface can have a significant effect on analyte mobility as a consequence of electroosmotic flow. Electroosmotic flow (EOF) is the bulk flow of the liquid electrophoresis medium which arises due to the effect of the electric field on counterions adjacent to the negatively charged channel wall. Because the channel wall is negatively charged under most pH conditions, there is a build-up of positive counterions (cations) in the solution adjacent to the wall. In an electric field, this cylindrical shell of cations causes the bulk flow of the medium to assume the character of a positively charged column of fluid which migrates toward the cathodic electrode at an EOF rate dependent on the thickness of the shell.
The rate of EOF can provide an important variable that can be optimized to improve the separation of two or more closely migrating species. In particular, when electrophoresis is carried out under conditions in which EOF and the migration of species to be separated are in opposite directions, the effective column length for separation can be made extemely long by making the rate of EOF in one direction nearly equal to the electrophoretic migration rate of the analyte attracted most strongly in the opposite direction by the electric field. A significant problem with using such conditions in capillary electrophoresis (CE) applications has been that the rate of EOF is highly sensitive to the nature and composition of the selected electrophoretic medium, as well as to the chemical condition of the capillary wall. That is, it has been difficult to sustain consistent migration times from run to run and from capillary to capillary due to chemical changes at the surface of the capillary wall after successive runs, and due to variability in the condition of the capillary walls of different capillary tubes from the same or different suppliers.